It might be the fastest. You can outwalk the boat ride if you go at a decent clip. The bus would be quicker, but the catch is you're leaving DHS right after Fantasmic...so is everyone else. If you catch the first bus to Epcot, you'll probably outpace walking, but no guarantee you'll be able to get on that bus.
One last thing...I assume for the night you're talking about that Epcot has EMH, otherwise, Epcot usually closes at 9.